Sydney's balmy autumn breaks temperature records

Sydney's remarkable spell of warm weather has been so prolonged that any reversion to long-term averages will feel almost chilly.

The city will cap a record month on Saturday with daily tops averaging about 23.2 degrees, eclipsing the previous warmest May - set in 1958 - by about half a degree, according to Bureau of Meteorology figures.

Unseasonable: Swimmers at Bondi Beach take advantage of another warm Autumn day.Credit:Anthony Johnson

This autumn was also Sydney's mildest, adding to last year's record warm winter and spring and making the past 12 months the city's hottest in 155 years, Weatherzone says.

While 2013 was the city's hottest calendar year, Sydney had probably not had a run of such exceptionally warm seasons since 1914, said Acacia Pepler, a climatologist for the bureau.

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Back then "we had the warmest autumn, the second warmest winter, equal warmest spring, and then the second warmest summer", Ms Pepler said.

1914 was also an El Nino year, which this year is also shaping up to be. During El Nino years, southern Australia, including most of NSW, can expect drier and hotter winters and springs, with the potential for droughts and active fire seasons.

The bureau's seasonal outlook released this week indicates all of NSW has a high chance of abnormal dry and warm conditions this winter.

While this weekend, particularly Sunday, should see the city's best rainfall in weeks, maximum temperatures are likely to be close to 20 degrees on both days and remain mild next week.

"We'll see the low 20s for Sydney" until next weekend, said Rob Sharpe, a meteorologist with Weatherzone. "But that's still 3 to 4 degrees above average for this time of year.''

South-eastern Australia was unlikely to see "a proper frontal system" bringing cold conditions for another three weeks, Mr Sharpe said. Ski resorts might have their worst start to a season since 1991, he said.

The abnormal warmth this month stretched across a wide arc from Brisbane through to Adelaide and most points in between. While so-called blocking high pressure systems over the Tasman are not uncommon at this time of the year, the duration of this autumn's event made it historic, the bureau said in a special climate statement released on Friday.

"An unusual feature of this event is that maximum and minimum temperatures have been above average to a broadly similar extent," the report said.

"Most previous blocking episodes in the cooler months have been associated with below-average minimum temperatures and frost over at least some of the region."

Sydney, for instance, had 19 days in a row above 22 degrees, ending on Wednesday. The previous stretch was just nine, in 1978 and 2007, with both previous series ending several weeks earlier in the season.

The city also registered its longest warm spell compared with historical averages - running at least 12 days - for any time of the year, said University of NSW researcher Sarah Perkins.